1. Field of the Invention
The present invention relates to a surface acoustic wave convolver for obtaining a convolution output utilizing non-linear interaction of plural surface acoustic waves.
2. Related Background Art
The surface acoustic wave convolver has become important as a key device in diffused or spread spectrum communication. Also various researches are being made for the application as a real-time signal processing device. For example the present inventors made a proposal for improving the efficiency of such convolver in the Japanese Patent Application No. 62-60466 and the Japanese Patent Application No. 62-220489 corresponding to the U.S. patent application Ser. No. 167,254.
FIG. 1 is a schematic view of a conventional surface acoustic wave convolver.
On a piezoelectric substrate 1, there are formed a pair of interdigital electrodes 2, and a central electrode 3 positioned therebetween. The interdigital electrodes are used for generating surface acoustic waves corresponding to input signals, while the central electrode 3 is used for propagating the surface acoustic waves generated by the interdigital electrodes in mutually opposite directions and for obtaining the output signal.
When signals F(t)e.sup.j.omega.t and G(t)e.sup.j.omega.t are respectively supplied to one and the other of the interdigital electrodes 2, two surface acoustic waves: ##EQU1## propagate in mutually opposite directions on the surface of the piezoelectric substrate 1, wherein v is the velocity of said acoustic waves, and L is the length of the central electrode 3.
On the propagation path, there is generated a component representing the product of said surface acoustic waves due to the non-linear effect, and said component is integrated within the central electrode 3 and is taken out to provide an output signal H(t): ##EQU2## wherein .alpha. is a proportional coefficient.
In this manner there can be obtained F(t) and G(t) from the central electrode 3.
In the above-explained conventional structure, however, when the signal emitted from either interdigital electrode 2 reaches the other or interdigital electrode through the central electrode 3, a part of said signal is reflected and again reaches the central electrode 3. Thus the signal emitted from the first-mentioned electrode 2 is superposed with the signal reflected from the other comb-shaped or digital electrode 2 on the central electrode 3, thus generating convolution as explained above. Such phenomenon is called self-convolution.
In this manner the prior technology has been associated with a drawback that an unnecessary signal resulting from such self-convolution overlaps with the desired convolution signals.